Current automotive bumper systems generally consist of a C-shaped beam which spans the vehicle front rails. Loads experienced by the beam in a low speed collision (typically 2.5 to 5.0 mph) are passed to the front rails of the vehicle. In order to prevent damage to the components protected by the bumper system, the kinetic energy is absorbed by the bumper system. Other components such as hydraulic attenuators, absorptive foams or egg-crating type parts are usually added to help dissipate system energy. Multi-step processing is required because the present bumper systems are made up of several different types of materials.
U.S. Pat. No. 4,762,352, for example, discloses a bumper assembly with a front made of fiber reinforce thermoplastic resin, a back up beam and a layer of foam between the face and the back up member. Each of these parts are made of different materials and requires a separate processing step to form the bumper.
U.S. Pat. Nos. 4,635,984, 4,586,739 and 4,616,866 teach a bumper construction having a semi-rigid resilient face and a rear mounting member. An impact absorbing foam is layered between the two pieces. During a collision, the outer face deforms and compresses the foam dissipating at least some of the energy of the impact.
U.S. Pat. No. 4,482,180 appears to disclose a bumper system which may comprise an I-beam having a resilient outer covering and one or more layes of an energy absorbing foam between the I-beam and the outer covering.
Finally, U.S. Pat. No. 4,762,352 discloses a bumper having an inner support beam with a resilient outer cover. An impact absorbing foam is placed between the beam and the cover with an air space between the cover and the foam. The back beam is a hollow tube made of fiber reinforced thermoplastic resin.
Each of these designs is made up of several different components each formed from different materials. This means that many different steps are required to produce a single unit of limited effectiveness. These multiple steps and different materials add significantly to the cost of producing each vehicle. In addition, they add to the weight of the vehicle reducing the fuel efficiency of the vehicle.
In addition, as seen in the patents discussed above, most of the currently produced composite bumper beams have cross sections which are generally "C" shaped or closed sections. The closed sections are generally square or rectangular in cross section. The use of a particular section is determined as much by production methods and material properties as it by the requirements of the design.
C-sections are particularly suited to materials that are relatively difficult to form due to such problems as material flow limitations which limit the complexity of the finished part. For example, in C-sections made from reinforced thermoplastic resins, the resin and reinforcement material flows smoothly and evenly because the section is continuous with few or no branches. If preforms are needed, they can be simple with the best possibility of accurate placement in the tool.
Hollow closed sections derive naturally from bumper sections made from welded channel or combined C-section shapes. See for example U.S. Pat. No. 3,779,592. These shapes provided the best torsional rigidity but are not the simplest or most cost effective beams to produce using high performance materials. Most high performance materials that are used in high speed production lines (e.g. one part produced every 30 to 90 seconds) required high forming pressures, high strength tooling or complicated machinery. Hollow sections have added complexity in that they require some means of forming an internal shape. This is usually accomplished by the use of tooling slides, tooling or casting cores, gas assisted molding or similar processes. These methods significantly increase the complexity of the molding process and result in a further increase in investment cost, cycle time or both.
I-beams are widely used in architecture because they can carry heavy loads per unit weight. These loads, however, must be central to the beam. I-beams cannot carry an eccentric load to the same extent that they can carry a central load. As vehicle bumpers often experience eccentric loads, the I-beams inability to handle these loads has prevented their being used in bumper applications. For example, the bumper disclosed in U.S. Pat. No. 4,482,180 disclosed the use of an I-beam as one of the supports which can be used in the practice of the invention. If an ordinary I-beam were used, however, one would expect the front wall to collapse if it encountered an eccentric load. This is because the patent does not teach or suggest the use of any reinforcement for the front wall.
Hollow beam sections, on the other hand, can carry eccentric and central loads since they also have sections that resist torsion. For central loads, I-beams and hollow beams of equal area have equal load capacity.
C-section bumper failure often occurs with some buckling of the horizontal walls. This indicates that the ultimate load for a C-section bumper is lower than the ideal load and is also difficult to predict. This lack of predictability is due to the fact that no specific buckling area is designed into the bumper. As a result, the design safety factor for a C-section bumper must be larger than for an I-beam section. The buckling of the C-section bumper can be reduced by the addition of material to the bumper. While this increases the stiffness of the bumper, it is not as cost effective as shifting to a hollow beam or I-beam construction.
A major disadvantage of both the C and closed sections is that two walls are used to carry the span load. For bumper beam applications, this means that each of the horizontal walls of the beam must be capable of withstanding a substantial portion of an eccentric load (high and low hits) therefore increasing beam weight. Increased beam weight means increased construction costs in the form of additional materials used and also reduces the fuel efficiency of the vehicle by adding to the overall weight of the vehicle.